JP3516875B2 - Interferometer - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an interference measuring device for measuring the surface shape of a sample by utilizing light interference,
In particular, the present invention relates to an interference measurement device that can easily remove a measurement error due to the influence of disturbance such as vibration or air fluctuation.

[0002]

2. Description of the Related Art Conventionally, as a device for measuring the surface shape of a sample, an interference measuring device utilizing light interference has been known. Such an interference measuring device measures the surface shape and the like of a sample in a non-contact and highly sensitive manner, and is widely applied in the fields of precision measurement and precision processing.

[0003]

By the way, in such an interferometric measuring apparatus, while highly sensitive measurement can be realized, it is easily affected by disturbance such as vibration or air fluctuation, which causes a measurement error. There is a problem that it is easy.

In order to solve such problems, various methods have heretofore been proposed. Of these, the simplest method is to photograph the interference fringes at a shutter speed that is sufficiently faster than the frequency of the vibration in question.

However, this method has a problem that it is very difficult to obtain a desired interference fringe pattern by one-time photographing, and a high-power laser is required as a light source, which is economically disadvantageous.

Another method other than this is a method of performing electrical feedback control on the light source, the optical member, etc. based on the detection result of the interference fringes. Specifically, for example, a semiconductor laser capable of continuous oscillation is used as a light source, the intensity change of the interference fringes due to vibration is detected, and the injection current of the semiconductor laser is feedback-controlled based on the detection signal, whereby A method of fixing (locking) the strength has been proposed. Further, the phase of the interference fringes is detected by a spatial filter detector, and the injection current of the semiconductor laser or the drive unit (PZT (Piez
A method has been proposed in which the phase of the interference fringes is fixed and is arbitrarily controlled by feedback-controlling the drive voltage of a piezoelectric element such as an electric transducer). Furthermore, a method is proposed in which the injection current of the semiconductor laser is feedback-controlled while detecting the phase of the interference fringe by lock-in detection of the interference fringe obtained by modulating the injection current of the semiconductor laser without using a spatial filter detector. Has been done.

However, these methods have a problem that the structure of the device becomes complicated and it is not possible to sufficiently cope with a large vibration due to the restriction of the response performance of the electric circuit device for feedback control.

The present invention has been made in consideration of the above points, and an object thereof is to provide an interference measuring apparatus capable of easily removing a measurement error due to the influence of disturbance such as vibration or fluctuation of air. And

[0009]

The present invention relates to an interference measuring apparatus for measuring the surface shape of a sample by utilizing the interference of light, a laser light source for projecting light on the sample, the laser light source, and the laser light source. A beam splitter arranged between the sample, a reference mirror arranged at a position different from the sample and reflecting light guided through the beam splitter, light reflected by the sample through the beam splitter, A detector for detecting an interference fringe pattern of interference light with the light reflected by the reference mirror through the beam splitter, the laser light source, the beam splitter and the reference mirror, in the sample through the beam splitter. Arranged so that the reflected light and the light that has passed through the beam splitter and reflected by the reference mirror returns to the laser light source through the beam splitter. The mode of the resonance wavelength of the composite resonator including the internal resonator formed by the laser light source and the external resonator formed by the beam splitter, the reference mirror, and the sample is adjusted according to the change of the optical path difference in the external resonator. The interferometer is characterized by being configured to shift.

According to the present invention, a part of the light respectively reflected by the sample and the reference mirror is returned to the light source through the beam splitter, and the internal resonator formed by the laser light source, the beam splitter, the reference mirror and the sample are provided. Since the mode of the resonance wavelength of the composite resonator composed of the external resonator and the optical resonator is configured to shift in accordance with the change of the optical path difference in the external resonator, the composite resonator composed of the light source and the other optical system. The resonance wavelength mode of shifts according to disturbance such as vibration and air fluctuation. As a result, the change in the phase of the interference fringes caused by the change in the optical path difference of the optical system can be suppressed to a small level. Therefore, the device can be used as a surface plate (optical bench).
Even in the case where it is not installed on the upper side, the measurement error due to the influence of disturbance such as vibration or air fluctuation can be easily removed with a simple structure.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram showing an embodiment of an interference measuring apparatus according to the present invention.

As shown in FIG. 1, the interference measuring apparatus 10 is
The surface shape of the concave surface 20a of the sample 20 is measured. The interferometer 10 includes a semiconductor laser (light source) 1 that projects a laser beam onto a sample 20, a beam splitter 2 arranged between the semiconductor laser 1 and the sample 20, the semiconductor laser 1 and the sample 20. A reference mirror 3 and a detector 4 are provided on one side and the other side of the beam splitter 2 on a straight line that is substantially orthogonal to the straight line connecting the two. Here, in the beam splitter 2, part of the laser light projected from the semiconductor laser 1 via the lens 7 can be guided to the sample 20 via the objective lens 8 and the rest can be guided to the reference mirror 3. It is like this. In the beam splitter 2, the laser light guided to and reflected by the sample 20 and the laser light guided to and reflected by the reference mirror 3 are combined to form interference light, a part of which is formed by the lens 9 While being guided to the detector 4 via the lens, the rest can be returned to the semiconductor laser 1 via the lens 7.

Further, the interference measuring device 10 is provided with an interference fringe analyzer 5 for analyzing the detection result of the detector 4. Here, the detector 4 detects the phase distribution (interference fringe pattern) of the interference light, and is, for example, a CCD (Charge).
Coupled Device). The interference fringe analyzer 5 is for generating a three-dimensional image corresponding to the surface shape of the concave surface 20a of the sample 20 based on the interference fringe pattern detected by the detector 4. For example, a CPU (central processing unit) On a general computer having a memory and a memory, a predetermined program stored in the memory is stored as C
It can be realized by executing in PU. The interference fringe analyzer 5 has a function of fitting the interference fringe pattern detected by the detector 4 to a reference surface (for example, a parabolic surface), and determines the position of the central axis of the concave surface 20a of the sample 20. The effect of defocus can be removed by correction.

Further, the interferometric measuring apparatus 10 includes a light quantity adjuster 6 arranged between the semiconductor laser 1 and the beam splitter 2 for adjusting the quantity of laser light returning to the semiconductor laser 1 via the beam splitter 2 and the lens 7. I have it. Here, the light quantity adjuster 6 transmits the laser light projected from the semiconductor laser 1 almost completely while the beam splitter 2
It is for adjusting only the amount of laser light returning to the semiconductor laser 1 via, and is composed of, for example, a polarizer capable of changing the polarization state.

The semiconductor laser 1, the beam splitter 2, the reference mirror 3, the detector 4, the light quantity adjuster 6 and the lenses 7 and 9 of the interferometer 10 are installed on the first table 11, and the sample 20 and the objective. Lens 8 is the second table 1
It is installed on 2.

Next, the operation of this embodiment having such a configuration will be described.

First, laser light is projected from the semiconductor laser 1 through the lens 7. This laser light is split into two in the beam splitter 2, part of which is guided to the sample 20 via the objective lens 8 and the rest of which is guided to the reference mirror 3.

Next, the laser beams guided to the sample 20 and the reference mirror 3 are reflected by the sample 20 and the reference mirror 3, respectively.

Thereafter, the laser beams reflected by the sample 20 and the reference mirror 3 are returned to the beam splitter 2,
In the beam splitter 2, these laser lights are combined to form interference light, a part of which is guided to the detector 4 via the lens 9, and the rest is reflected by the lens 7 and the light quantity adjuster 6.
It is returned to the semiconductor laser 1 via.

The interference fringe pattern detected by the detector 4 is input to the interference fringe analyzer 5, and the interference fringe analyzer 5 generates a three-dimensional image corresponding to the surface shape of the concave surface 20a of the sample 20. .

By the way, in such an interference measuring apparatus 10, the semiconductor laser 1, the beam splitter 2, the reference mirror 3, the detector 4, the light quantity adjuster 6, which are installed on the first table 11 and the second table 12, When a disturbance such as vibration is applied to the lenses 7, 8, 9 and the sample 20, the optical path length of the laser beam returning to the beam splitter 2 via the sample 20 and the laser beam returning to the beam splitter 2 via the reference mirror 3 are detected. The optical path length fluctuates, which causes the beam splitter 2
The phase of the interference fringe pattern formed in 1 changes.
However, in the interferometer 10 shown in FIG. 1, a part of the laser light is returned to the semiconductor laser 1 via the beam splitter 2 to fix (lock) such a change in the phase of the interference fringe pattern. You can

The principle of the interference fringe fixing (locking) phenomenon in the interference measuring apparatus 10 shown in FIG. 1 will be described below with reference to FIGS. FIG. 2A shows a system of the interferometer 10 shown in FIG. 1 as a multilayer film model. The internal resonator 30 corresponds to the semiconductor laser 1 and other optical systems (beams) other than the semiconductor laser 1. The external resonator 40 corresponds to the splitter 2, the reference mirror 3, and the sample 20, etc.) to form a composite resonator. FIG. 2B is an equivalent resonator model of the system shown in FIG. 2A, in which a resonator (Fabry) obtained as a steady-state solution using boundary conditions of each optical member (mirror, beam splitter, etc.).・ Perot resonator). 2A and 2B, n, g, and L are the refractive index, the gain, and the resonator length of the internal resonator 30, respectively. Also, ρ, ρ ′, ρ ₀ , ρ ₀ ′,
ρ _e , ρ ₁ and ρ ₂ are the electric field reflection coefficients at each optical member, τ,
τ ′, τ ₀ , τ ₀ ′ are the transmission coefficients of each optical member.
Further, L ₁ and L ₂ are optical path lengths of laser light in the external resonator 40.

In the model shown in FIG. 2B, the two optical path lengths L of the external resonator 40 are taken into consideration in consideration of the resonance condition of the resonator.
_When the relationship between the variation amount Δd of the difference (optical path difference) between ₁ and L ₂ and the resonance wavelength (oscillation wavelength) λ is obtained, it is found that the oscillation wavelength λ changes periodically corresponding to the change of Δd. The cycle of such a change is approximately half the oscillation wavelength λ.

Here, a large mode hop occurs within such one cycle. This mode hop interval is 0.
23 nm, which corresponds to the mode spacing of the internal resonator 30. In the interferometer 10 shown in FIG. 1, the relationship between the fluctuation amount of the optical path difference and the oscillation wavelength (laser wavelength) was measured by a wavelength meter (OSA: Optical Spectrum Analyzer), and it was in good agreement with the above-described calculation formula. Was there.

Further, a finer mode hop occurs within such one cycle. Wherein the spacing of fine mode hopping is 0.0007Nm, which corresponds to the mode spacing of the external resonator 40, i.e. ^{Δλ = λ 2/2 (L} 2 -L 1) = 0.0007nm .

The phase of the interference fringe pattern corresponding to the difference (optical path difference) between the _two optical path lengths L ₁ and L ₂ of the external resonator 40 follows the movement of each optical element due to disturbance such as vibration. Although changing, the phase of this interference fringe pattern is canceled by the above-mentioned mode hop. That is, the resonance wavelength of the composite resonator shifts in accordance with the change in the optical path difference of the external resonator 40, and thus the change in the phase of the interference fringe pattern is suppressed to be smaller than 2π. When the change in the phase of the interference fringe pattern was calculated by considering the mode hop, it was found that the variation in the phase of the interference fringe pattern could be suppressed within 0.2π.

As described above, according to this embodiment, the sample 2
0 and a part of the laser light reflected by the reference mirror 3 returns to the semiconductor laser 1 via the beam splitter 2, so that the semiconductor laser 1 and other optical systems (the beam splitter 2, the reference mirror 3) follow the principle described above. And sample 20
And the like), the mode of the resonance wavelength of the composite resonator is shifted in accordance with the disturbance such as vibration or air fluctuation. As a result, the change in the phase of the interference fringes caused by the change in the optical path difference of the optical system can be suppressed to a small level. Therefore, even when the device is not installed on the surface plate (optical bench). The measurement error due to the influence of disturbances such as vibrations and air fluctuations can be easily removed with a simple structure.

Further, according to the present embodiment, since the light quantity adjuster 6 for adjusting the quantity of the laser light returning to the semiconductor laser 1 via the beam splitter 2 and the lens 7 is provided, the arrangement state of the optical system and the The amount of laser light returning to the semiconductor laser 1 can be set to an optimum state according to the operating state of the optical member, etc. Therefore, the measurement error due to the influence of disturbance such as vibration or air fluctuation can be accurately and flexibly made. Can be removed.

Further, according to the present embodiment, the sample 20
And the objective lens 8 on the same table (second table 1
2) Since it is installed above, the positional relationship between the concave surface 20a and the objective lens 8 can be kept constant even when vibration or the like occurs in the optical axis direction (horizontal direction in the drawing). The amount of laser light returning to the semiconductor laser 1 can be kept constant at all times. Therefore, the phenomenon of mode hopping that changes depending on the amount of laser light returning to the semiconductor laser 1 is stabilized, and vibrations and air fluctuations are stabilized. It is possible to stably remove the measurement error due to the influence of disturbance such as.

In the above-described embodiment, the arrangement of the optical system of the interferometer 10 is the same as that of the Twyman-Green interferometer. It is also possible to

Further, although the semiconductor laser is used as the light source in the above-mentioned embodiments, any light source such as a CO ₂ laser may be used as long as it is a light source that can utilize the above-mentioned interference fringe locking phenomenon. it can.

[0032]

EXAMPLES Next, specific examples of the above-described embodiment will be described.

In this embodiment, in the interferometer shown in FIG. 1, the oscillation wavelength of 690.0n is used as the light source.
The surface shape of a sample having a concave surface with a radius of curvature of 150 mm and a diameter of 130 mm was measured using a semiconductor laser of m and an oscillation output of 30 mW. The main body of the interferometer (semiconductor laser, beam splitter, reference mirror, detector, light quantity adjuster and lens) was installed on a wooden desk. The sample and the objective lens were fixed on the same base and placed on a desk separate from the above desk. And
In this state, by adjusting the arrangement state of each optical member and adjusting the light quantity adjuster, in the beam splitter, the laser light guided to the sample and reflected and the laser light guided to the reference mirror and reflected At least 0.1% of the interference light is returned to the semiconductor laser.

FIG. 3 is a diagram showing a result of measuring the surface shape of the concave surface of the sample according to this embodiment, which is a PV value (Peak-Val).
3D display of the distribution of the ley value).

As shown in FIG. 3, the PV value of the concave surface shape of the sample was about λ / 6, and the reproducibility was about λ / 100. Also, its rms value (root-mean-square v
alue) was about λ / 21, and the reproducibility was about λ / 400. This result is almost the same as the result of measuring the same sample on the surface plate (optical bench), and there is no measurement error even when the interferometer is installed on a wooden desk that is not the surface plate. It turned out that various measurements are possible.

[0036]

As described above, according to the present invention, even when the interference measuring device is not installed on the surface plate (optical bench), the measurement is performed by the influence of disturbance such as vibration or air fluctuation. The error can be easily removed with a simple structure.

[Brief description of drawings]

FIG. 1 is a diagram showing an embodiment of an interference measuring apparatus according to the present invention.

FIG. 2 is a diagram for explaining the principle of the interference fringe locking phenomenon in the interferometer according to the present invention.

FIG. 3 is a diagram showing a specific measurement result by the interference measuring device shown in FIG.

[Explanation of symbols]

1 Semiconductor laser (light source) 2 beam splitter 3 reference mirror 4 detector 5 Interference fringe analyzer 6 Light intensity controller 7,8,9 lens 10 Interferometer 11,12 tables 20 samples 20a concave

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Claims (4)

(57) [Claims] 1. An interference measuring apparatus for measuring the surface shape of a sample by utilizing the interference of light, comprising: a laser light source for projecting light onto the sample; and a laser light source disposed between the laser light source and the sample. A beam splitter, a reference mirror arranged at a position different from the sample and reflecting light guided through the beam splitter, a light reflected by the sample through the beam splitter, and a reference mirror passing through the beam splitter. A detector for detecting an interference fringe pattern of interference light with the reflected light, the laser light source, the beam splitter and the reference mirror, the light reflected by the sample through the beam splitter, and the beam The laser light source is arranged so that the light passing through the splitter and reflected by the reference mirror returns to the laser light source through the beam splitter. A mode in which a resonance wavelength of a composite resonator including a local resonator, the beam splitter, the reference mirror, and an external resonator formed by the sample is shifted according to a change in optical path difference in the external resonator. An interferometer which is characterized in that 2. The light source controller according to claim 2, further comprising a light amount adjuster disposed between the laser light source and the beam splitter to adjust an amount of light returning to the laser light source via the beam splitter. The interferometer according to 1. 3. The interferometer according to claim 1, wherein the laser light source is a semiconductor laser. 4. The laser light source, the beam splitter, and the reference mirror are installed on a first table, and the sample is installed on a second table. The interferometer according to any one of 1.